This invention relates to a condenser microphone which can be operated without feeding from the outside, in that electric charges are injected from the outside to electret (polarization), and production method thereof.
As is generally well known, the condenser microphone is structured in such a manner that it has a conductive diaphragm and a conductive fixed electrode (hereinafter, called conductor fixed electrode) oppositely arranged through the air layer in parallel with the conductive diaphragm, and the change of the capacitance between the conductive diaphragm and the conductor fixed electrode due to the vibration of the diaphragm is detected as the sound pressure on the diaphragm. In this case, in order to output the detection as an electric signal, between two conductors (conductive diaphragm and the conductor fixed electrode), the DC voltage is previously supplied to form the positive potential, and as its change, the change of the capacitance is detected as the electric signal. Further, it is well known that the magnitude of the output voltage to a unit sound pressure, that is, the sensitivity, is proportional to the applied DC voltage.
The positive potential is, although there is a case where it is supplied from the outside DC power source, recently, a method to use so-called electret (dielectric at least one portion of which is polarized) by which the dielectric film such as FEP (fluoro ethylene propylene) is attached to any opposing surface of, for example, the conductive diaphragm or conductor fixed electrode, and the electric charge is injected into and fixed onto the dielectric film, and from the electric field formed by the electric charge, the electric potential corresponding to the positive voltage is obtained, is developed, Thereby, the condenser microphone in which the outside DC power source is not necessary, is put to practical use.
Next, referring to FIG. 5, this electret type condenser microphone will be described.
In FIG. 5, in the inside of the housing 101, a diaphragm plate ring 102, electret forming-use dielectric 104 which is a portion of the diaphragm, spacer 105, fixed electrode 106, and insulation ring 107 are provided. This dielectric 104 is structured by a thin film such as FEP, and on the outer surface, the metal such as gold or nickel is attached by evaporation, and the film on which the function as the earth electrode 103 is given is provided. On the fixed electrode 106, a vent hole 106A is formed. Further, to this fixed electrode 106, the output terminal 108 is connected, and the potential difference (voltage) between the housing 101 and it, is obtained.
As the injection method of the electric charge into the dielectric 104 for the electret formation, each kind of method in which an electron beam is used or corona discharge is used, is proposed. Further, when the injection of the electric charge is conducted by these methods, there is some difference in the depth into which the electric charge is injected depending on the adopted method. For example, in the electron beam, although the injection can be conducted deeply to some degree, also by the method, the electric charge to be injected is fixed to very shallow portion of several tens xcexcm at the most from the surface.
That is, this is for the reason why, when dielectric film for forming the electret is used as the conductive diaphragm, generally, the conductor metal is formed on its outside as the above earth electrode by a method of the evaporation, the film thickness of about several tens xcexcm. In many case, as the conductor metal, the heavy metal such as nickel or gold is used. However, even when ion irradiation is conducted, of course, by the high temperature electric field method, or by the method of the electron beam, ion flow, or other method, the ion can not penetrate the film of the conductor metal for which such the heavy metal is used.
Accordingly, when the injection electric charge is fixed in very shallow portion in this manner, when, with this surface, the electrolyte such as, for example, the water, the conductor such as the metal, or non-insulating body such as the human skin is brought into contact, there is a possibility that it is easily discharged through the shallow distance, and the injected electric charge is dissipated, and the function as the microphone is destroyed or damaged.
Accordingly, as the conventional electric charge injection method, generally, it is conducted from the surface on the opposite side of the surface onto which this conductor metal is attached, that is, from the inner surface side of the dielectric 104 facing the air layer 109.
From such the conditions, conventionally, the electric charge injection is conducted before the product is assembled as the microphone, and the electret is previously formed as a part, and it is necessary that, by using this, the assembling is conducted. As the result, when the non-insulating body such as the human body is brought into contact with the formation surface of the electret, or it is exposed to the excessive humidity during the conservation as the part, the electric charge injected at no small pains is discharged, and there is a problem that the performance as the microphone after the assembly is spoiled.
Accordingly, in view of the above conditions, the object of the present invention is to provide a high reliable condenser microphone and its production method by which the injection of the electric charge can be conducted from the metal coating surface after the assembly of the microphone, not only the assembly becomes easy, but after the dielectric is sufficiently washed, it is maintained in the hermetically sealed condition, and is strong to the water or humidity, and even when the environmental condition such as the humidity or temperature is severe, the excellent electric sound conversion characteristic can also be maintained.
According to the first aspect of the invention, a condenser microphone comprises: a conductive diaphragm having an earth electrode layer formed of a conductive light metal; a conductor fixed electrode arranged opposite to the conductive diaphragm through an air layer; an organic dielectric layer formed of the organic compound provided on the boundary surface side between the air layer and the conductive diaphragm; and a permanent electric charge layer composed of ions or electrons formed on the earth electrode layer side from a middle position in the thickness direction of the organic dielectric layer in the inside of the organic dielectric layer.
Thereby, the electric field can be injected by penetrating the metallic surface of the earth electrode layer, for example, like that the corona discharge is generated in the air and ions are formed and accelerated by the electric field.
Further, in the present invention, the inside of the organic dielectric layer from the permanent electric charge layer to the inner end surface contacting with the air layer is made to the electret, and has the electric potential.
Thereby, without feeding the power from the outside, the condenser microphone can be operated.
Further, in the present invention, the organic dielectric layer has 1-50 xcexcm film thickness, and it is preferable that the earth electrode layer is formed by making the conductive light metal into the film of not larger than 0.1 xcexcm thickness on the organic dielectric layer.
Further, in this invention, the organic dielectric layer may be formed by using any one of FEP (fluoro ethylene.propylene), PFA (polyfluoro.acetal), and PTFE (polytetra.fluoroethylene)
According to the second aspect of the invention, a condenser microphone comprises: a conductive diaphragm having an earth electrode layer formed of a conductive light metal; a conductor fixed electrode arranged opposite to the conductive diaphragm through an air layer; an inorganic or organic dielectric layer formed of the inorganic oxide or the organic compound provided on the boundary surface side between the air layer and the conductor fixed electrode; and a permanent electric charge layer composed of ions or electrons formed on the conductor fixed electrode side from the middle in the thickness direction of the inorganic or organic dielectric layer in the inside of the inorganic or organic dielectric layer.
Thereby, the electric field can be injected by penetrating the metallic surface of the earth electrode layer, like that, for example, the corona discharge is generated in the air, and ions are formed and accelerated by the electric field.
Further, the present invention can be structured in such a manner that the inside of the inorganic or organic dielectric layer from the permanent electric charge layer to the inner end surface contacting with the air layer is made to the electret, and has the electric potential.
Thereby, without feeding the power from the outside, the condenser microphone can be operated.
Further, in this invention, the dielectric layer whose thickness is not larger than 4 xcexcm, formed of the organic compound is used for the conductive diaphragm, and it is preferable that the earth electrode layer is provided by making the conductive light metal into the film of not larger than 0.1 xcexcm thickness on the organic dielectric layer.
Further, in this invention, the silicon dioxide (SiO2) of not smaller than 1 xcexcm thickness can be used for the inorganic or organic dielectric layer.
Further, in this invention, it is preferable that a circular hole whose inner diameter is not smaller than 1.0 mm is formed on the outer surface of a housing which accommodates the conductive diaphragm and the conductive fixed electrode.
According to the third aspect of the invention, a production method of the condenser microphone which has the conductive diaphragm having the earth electrode layer formed of the conductive light metal, the conductor fixed electrode arranged opposite to the conductive diaphragm through the air layer, the organic dielectric layer formed of the organic compound provided on the boundary surface side between the air layer and the conductive diaphragm, and the permanent electric charge layer composed of ions or electrons formed in the inner portion receding from the air layer side to the middle position in the thickness direction of the organic dielectric layer in the inside of the organic dielectric layer, the production method of the condenser microphone comprises the steps of: after the condenser microphone is assembled, injecting ionized objects or electrons accelerated into the organic dielectric layer from the outside of the conductive diaphragm to form the permanent electric charge layer.
Thereby, not only the assembly becomes easy, but after the organic dielectric layer is sufficiently washed, it is maintained in the hermetically sealed condition, and is strong to the water or humidity, and even under the condition that the environmental condition such as the humidity or temperature is severe, the excellent electric sound conversion characteristic can also be maintained.
According to the fourth aspect of the invention, a production method of the condenser microphone which has the conductive diaphragm having the earth electrode layer formed of the conductive light metal, the conductor fixed electrode arranged opposite to the conductive diaphragm through the air layer, the inorganic or organic dielectric layer formed of the inorganic oxide or the organic compound provided on the boundary surface side between the air layer and the conductive diaphragm, and the permanent electric charge layer composed of ions or electrons formed in the inner portion receding from the air layer side, from the middle in the thickness direction of the inorganic or organic dielectric layer in the inside of the inorganic or organic dielectric layer, the production method of the condenser microphone comprises the steps of: after the condenser microphone is assembled, injecting ionized objects or electrons accelerated into the inorganic or organic dielectric layer from the outside of the conductive diaphragm to form the permanent electric charge layer.
Thereby, not only the assembly becomes easy, but after the inorganic dielectric layer is sufficiently washed, it is maintained in the hermetically sealed condition, and is strong to the water or humidity, and even under the condition that the environmental condition such as the humidity or temperature is severe, the excellent electric sound conversion characteristic can also be maintained.